All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae). Every year this disease leads to millions of dollars of lost lettuce crop throughout the world. Downy mildew (Bremia lactucae) is highly destructive on lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, Bl:17, Bl:18, Bl:20, Bl:21, Bl:22, Bl:23, Bl:24, Bl:25, Bl:26, Bl:27, Bl:28, Bl:29, Bl:30, Bl:31, Bl:32 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants); IBEB press release “New race of Bremia lactucae Bl:27 identified and nominated”, May 2010; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), “New race of Bremia lactucae Bl:28 identified and nominated”, March 2011; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB press release, “New races of Bremia lactucae, Bl:29, Bl:30 and Bl:31 identified and nominated”, August 2013; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB press release, “A new race of Bremia lactucae, Bl:32 identified and nominated in Europe”, May 2015), Ca-I, Ca-IIA, Ca-IIB, Ca-III, Ca-IV (Schettini, T. M., Legg, E. J., Michelmore, R. W., 1991. Insensitivity to metalaxyl in California populations of Bremia lactucae and resistance of California lettuce cultivars to downy mildew. Phytopathology 81(1). p. 64-70), and Ca-V, Ca-VI, Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.” In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, Calif., pp. 55-68).
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
Mature romaine lettuce is mainly harvested by hand. This harvesting process is labor-intensive, and relatively laborer-unfriendly. As it getting more and more costly to hire laborers that want to work outdoor in the field, close to the ground and under all kinds of adverse weather conditions, there is strong interest of the lettuce industry to mechanize this manual harvesting process. However, until now the lack of uniform quality among mature plants is a major problem for mechanical harvest. Poor quality of basal leaves, internal breakdown of heartleaves due to tipburn, fringe burn of leaf margins and fast bolting are the main quality problems. If one would like to mechanize the harvest process, these quality problems result in additional hand sorting, which is costly, and/or a very low net yield, if the plant parts with a high risk of quality problems are mechanically discarded. The latter approach is chosen by some producers that produce romaine hearts, i.e. the intact yellow-green heart of the romaine lettuce without the outer leaves. For producers that produce romaine lettuce for processing, i.e. pre-washed and pre-cut leaves, this is not feasible because of four major constraints.
The first is the requirement for a mix of colored leaves, which requires the preservation of the undamaged outer leaves. The second constraint is the strongly reduced net yield, which is a result of the fact that an automated knife should cut through the leaf but not through the core. Especially fast bolting lettuce varieties are unacceptable for mechanized harvest of romaine lettuce for processing purposes because of their long core.
The third constraint is caused by the use of mechanically-driven horizontal knives that easily damage the hollow main veins of romaine leaves and cut halfway through the prostrate, round-shaped, lower outer leaves, which results in wide cut surfaces. Cutting damage on a leaf with a hollow main vein is often not restricted to the cut surface but bruising extends into the leaf along the main vein.
The fourth constraint is the requirement for a leaf to have a good shelf life. This means the leaves should get through the washing and drying process in the factory without bruising or breaking. In addition, the leaves should be strongly blistered to avoid sticking to each other and to the plastic of the packaging, which increases local humidity on the leaf and reduces local respiration.
At present the attractive red color in pre-packed lettuce mixtures is often provided by plant species other than Lactuca sativa. Use is made of radicchio rosso (Cichorium intybus), red cabbage or red-veined spinach or red-veined chard. The disadvantage of using leaves from other vegetables than lettuce is that the different taste of these other vegetables is often experienced as undesirable. Radicchio rosso is for example a Cichorium intybus that has a bitter taste. For taste, red-leaved lettuce (Lactuca sativa) is usually preferred over other red-leaved species.
The red leaf color of lettuce is caused by anthocyanin in the leaves. An intensely red colored leaf corresponds with a very high concentration of anthocyanin in that leaf. Anthocyanin is a highly desirable antioxidant, and as such, is attractive to consumers for its potentially natural health properties.
When lettuce plants reach the bolting stage, an elongated stalk with flowers grows from within the main stem of the lettuce plant. Once lettuce plants have reached the bolting stage, the leaves produce a bitter taste that is not desirable by consumers. Bolting is thus a very undesirable trait in the production of leafy vegetables such as lettuce. Therefore, slow bolting varieties are very much desired by growers.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.